1. Field of the Art
This invention relates to an electronic endoscope system suitable for applications in medical and industrial fields, and more particularly to an electronic endoscope system incorporating an image sensor drive compatible with solid-state image sensor elements of different classes in sensitivity.
2. Prior Art
The electronic endoscopes usually incorporate a solid-state image sensor element like CCD at the tip end of a catheter portion to be inserted into an internal portion of a human body or of a machine, thereby picking up images of an internal portion of interest and displaying its color images on a monitor screen for observation purposes.
In this connection, to cope with the darkness of intracorporeal or internal portions to be examined or diagnosed through the endoscope, the locality of interest which confronts the inserted tip end of the catheter portion is observed in an illuminated state. For this purpose, the endoscope is connected to a light source, and illuminating light rays from the light source are led through a light guide, which is coextensively provided in the catheter portion, and projected through an illumination window at the tip end of the catheter portion. Further, an observation window with an objective lens is provided also at the tip end of the catheter portion to focus the image of an intracorporeal portion under observation on a solid-state image sensor element through the objective lens. As well known in the art, the solid-state image sensor element has a multitude of pixels which accumulate signal charges when the image sensor is exposed to light. The accumulated signal charges in the respective pixels are sequentially read out in a predetermined timing and sent to a signal processing unit to obtain video signals.
In this connection, for an endoscopic image pickup system using a solid-state image sensor, it is the usual practice to adopt a single-element image sensor for the purpose of minimizing the diameter of the catheter portion of the endoscope. Besides, for improving the resolution of the picture images, the image sensor element is driven by a sequential color scan system which sequentially fetches the picture signals of red (R), green (G) and blue (B) fields. Generally, the field signals of these three colors are processed into a composite video signal thereby to display a color image on a monitor screen.
For these purposes, the illumination light rays projected from the light source through the light guide needs to change its spectral characteristics to irradiate an internal area of interest sequentially with light rays of R, G and B wavelengths. Further, for reading out the accumulated signal charges from the solid-state image sensor element, it is necessary to provide a blank period between the irradiation periods of the respective colors. To this end, the light source usually incorporates a color filter, including a color wheel which is located in the light path between a source lamp and an input end of the light guide, and a drive means which rotationally drives the color wheel. The color wheel is provided with a filter track containing a R-filter zone transmissive of light of red wavelength, a G-filter zone transmissive of light of green wavelength and a B-filter zone transmissive of light of green wavelength successively and alternately with a blank light-blocking zone.
When the color wheel is driven to rotate, picture signals of R field are accumulated in the solid-state image sensor element in a red irradiation period when the R-filter zone is in the path of the illuminating light, and these signals are read out in a succeeding blank period. Similarly, picture signals of G field are accumulated in a green irradiation period when the G-filter zone is in the illuminating light path, and read out in a succeeding blank period. Picture signals of B field are accumulated in the next blue irradiation period when the B-filter zone comes into the illuminating light path, and the accumulated signals are read out in a succeeding blank period. These operations are repeated cyclically to obtain video signals of the internal portion under observation.
In an actual image pickup operation using such a solid-state image sensor element, for example, the light rays of RGB wavelengths are successively and periodically projected from the light source at time intervals of 16.6 ms while exposing the solid-state image sensor to reflected light and reading out signal charges in each blank period when a light-blocking zone comes into the illumination light path. After signal processing, the read-out picture signals are stored in a field memory which is arranged to renew one of picture data of R, G or B field each time when new field data of the corresponding color are received. The renewed picture data of one color are read out from the field memory concurrently with previous field data of the other two colors, and converted into concurrent video signals by a color encoder. The output video signals of the color encoder consist of odd-numbered field signals and even-numbered field signals. In this regard, the endoscope solid-state image sensor element has storage portions which correspond approximately to 1/2 of the number of pixels in the light receiving areas in the vertical direction. Therefore, in reading out field data of one color, it is the usual practice to add up two pixels in the vertical direction by mixed pixel read-out.
Generally, when mixing two pixels in the vertical direction in connection with the interlacing action, an odd-numbered field signal or an even-numbered field signal is obtained by adding a preceding line or a succeeding line to each scanning line, permitting to reproduce a picture image by line interlacing on the basis of these two field signals. However, in case of an electronic endoscope, it has been the general practice to read out only one of these two field signals, for example, only the odd-numbered field signals.
Further, in consideration of insertion into intracavitary portions of human body, the catheter portion of the endoscope is desired to be as small as possible in diameter to ensure smooth passage through constricted portions and to lessen pains on the part of the patient. Therefore, the solid-state image sensor element like CCD to be incorporated into a tip end portion of the catheter as an image pickup means has been facing strong demands for reductions in size. In order to meet these demands by the use of a recently developed high-sensitivity solid-state image sensor element which has almost doubled sensitivity as compared with the conventional counterpart along with excellent dynamic range and RGB spectral sensitivity ratios, the present inventor proposed in his co-pending U.S. patent application Ser. No. 07/748,961 an electronic endoscope system capable of obtaining pictures of high quality from a solid-state image sensor element of a reduced size which is approximately halved in the number of pixels in the vertical direction as compared with the conventional counterpart and which is driven at a higher speed to produce two fields of picture data in the image pickup system within a period of one field in the image reproducing system.
In picking up an image of a subject, the high sensitivity image sensor element of this sort requires to repeat each of the RGB irradiations for a couple of times by the use of a color wheel of an arrangement different from the one which is usually used with an ordinary or normal sensitivity solid-state image sensor element. In this regard, since it is inconvenient and troublesome to change the light source according to the type of the solid-state image sensor element, the present inventor developed a rotary color filter which is provided with two or more filter tracks concentrically on a color wheel in such a way as to shift selected one of the filter tracks into a position in alignment with the path of illuminating light from the source lamp.
The rotary color filter of this arrangement can cope with a number of solid-state image sensors of different type, but needs to provide a wheel shifting means for moving the color wheel in a direction perpendicular to the illumination light path. The wheel shifting means is required to include not only a mechanism for shifting the color wheel position but also a mechanism for simultaneously shifting the position of a rotational drive motor for the color wheel, resulting in a substantial increase in size and complication of the rotary color filter construction as a whole.